The present invention relates to corona charging devices, particularly as used for discharging electrostatic images.
Corona charging devices in the form of thin conducting wires or sharp points are well known in the prior art. Illustrative U.S. Pat. Nos. are P. Lee, 3,358,289; Lee F. Frank, U.S. Pat. No. 3,611,414; A. E. Jvirblis, U.S. Pat. No. 3,623,123; P. J. McGill, U.S. Pat. No. 3,715,762; H. Bresnik, U.S. Pat. No. 3,765,027; and R. A. Fotland U.S. Pat. No. 3,961,574. Such devices are used almost exclusively in electrostatic copiers to charge photoconductors prior to exposure as well as for discharging. Standard corona discharges provide limited ion currents. Such devices as a rule achieve a maximum discharge current output on the order of 10 microamperes per linear centimeter, and require driving voltages on the order of tens of thousands of volts to achieve this output. At lower voltages these devices produce little or no output current. Additionally, corona wires are small and fragile, and easily broken. Because of their high operating potentials, they collect dirt and dust and must be cleaned or replaced frequently, in order to avoid the emission current fall off.
Corona discharges which enjoy certain advantages over standard corona apparatus are disclosed in Sarid et al., U.S. Pat. No. 4,057,723; Wheeler et al., U.S. Pat. No. 4,068,284; and Sarid, U.S. Pat. No. 4,110,614. These patents disclose various corona charging devices characterized by a conductive wire coated with a thick dielectric material, in contact with or closely spaced from a further conductive member. Various geometries are disclosed in these patents, all fitting within the above general description. These devices utilize an alternating potential in order to generate a source of ions, and a DC extraction potential. The patents disclose a preferred biasing range of 2000-6000 volts, relatively high values which are required in order to obtain significant extraction currents and therefore higher charging rates. These current outputs are exponential in character, in contrast to the fairly linear outputs of the present invention. U.S. Pat. No. 4,155,093 discloses ion generating apparatus which may be used for charge neutralization as well as deposition of net charge. This apparatus, which is difficult to fabricate, does not provide the high charging rates of the present invention.
Accordingly, it is a principal object of the invention to provide charging devices employing corona discharging which have superior performance compared with Prior art corona devices.
Another object of the invention is to provide a corona charging device which achieves high current densities. A related object is the achievement of high charging rates. Another related object is the avoidance of high biasing potentials in providing such charging rates.
A further object of the invention is to provide a charging device having a rugged and compact structure. A related object is to provide a device having a longer operational life than is customary in corona ion generators.
Still another object of the invention is the avoidance of emission current fall-off as the ion generator becomes slightly dirty. A related object is the achievement of uniform emission currents. Yet another object of the invention is the provision of a corona charging device with a reliable output potential.